PROTEIN. 



169 



very similar to that already mentioned when 

 speaking of albumen : in the milk, which is 

 the sole food on which the young of most ani- 

 mals subsist, no other protein compound has 

 been detected ; but no sooner has it become 

 the food of the young animal which it is in- 

 tended to nourish, than it is for the most part 

 converted into fibrin and albumen, thus fur- 

 nishing blood and muscle, together with most 

 of the other tissues of the body, which, though 

 less directly, are scarcely less certainly products 

 of the decomposition of this substance. The 

 composition of casein is represented by the for- 

 mula C 400 H 310 N 60 120 S, or ten equi- 

 valents of protein united to one equivalent of 

 sulphur, thus differing from fibrin and albu- 

 men in not containing any phosphorus. 



There is another modification of protein, 

 very similar to casein in its properties and 

 composition, which has been called both glo- 

 bulin and crystalline, from the circumstance 

 that it is found surrounding the blood globules 

 and also in the crystalline lens of the eye. It 

 appears to contain no phosphorus and less sul- 

 phur than casein, and is composed, according 

 to Mulder, of fifteen equivalents of protein 

 united to one of sulphur. 



The form in which protein exists in hair, 

 horn, nails, and the epidermis, and called by 

 Simon keratine, has been but imperfectly ex- 

 amined. That these substances are composed 

 chiefly of protein is proved by the circumstance 

 that if a solution of them be made in caustic 

 potash and neutralized with acetic acid, a co- 

 pious precipitate of protein is thrown down. 

 It is probable that other modifications of protein 

 will hereafter be found to exist in the animal 

 body, but those which I have now described 

 are all which have hitherto been detected. 



The animal body, however, is not the only 

 source from which protein and its compounds 

 are to be obtained. The researches of modern 

 chemists have led to the interesting fact that 

 they exist in the vegetable kingdom also, and 

 that they are there so extensively disseminated 

 that not a leaf, a seed, or a twig, in any of the 

 various tribes of plants, is free from them ; and 

 it is highly probable that the whole of the 

 protein compounds constituting the bodies of 

 animals are derived from plants. In the pre- 

 sent state of analysis it is perhaps too much to 

 say that the forms in which we find protein in 

 vegetables are absolutely the same, with regard 

 to the minute quantities of sulphur and phos- 

 phorus, as those found in animals ; but as far 

 as we are able to judge from similarity of pro- 

 perties, we may safely divide them in the same 

 way as the analogous animal principles ; viz. 

 into vegetable fibrin, vegetable albumen, and 

 vegetable casein. They all yield, when heated 

 with strong hydrochloric acid, blue or purple 

 solutions ; and when they are digested with a 

 solution of potash, and neutralized with acetic 

 acid, protein is invariably produced. 



Vegetable fibrin is found most abundantly 

 in the seeds of the cerealia, as wheat, oats, &c. : 

 it is also found dissolved in the juice of most 

 plants, especially that of grapes, carrots, tur- 

 nips, and beetroot, from which it shortly sepa- 



rates in the form of a flocculent precipitate 

 when taken from the plant and allowed to 

 stand. The readiest way of preparing it is to 

 knead vvheaten flour into a paste with water, 

 and then wash it on a linen cloth with a stream 

 of cold water until the whole of the starch is 

 removed, which is known by the water passing 

 through quite clear : the viscous mass which 

 remains on the cloth is subsequently purified 

 by washing with alcohol and ether, in both of 

 which the fibrin is insoluble. When dry it is 

 a hard horny-looking substance, semitranspa- 

 rent, without taste or smell, and sufficiently 

 heavy to sink in water, in which it is insoluble. 

 Phosphoric and acetic acids readily dissolve it ; 

 and it is reprecipitated in the form of white 

 flocks from its acid solution by carbonate of 

 ammonia and ferrocyanide of potassium, and 

 yellowish by tincture of galls; it is also preci- 

 pitated by bichloride of mercury and some 

 other metallic salts. It is perfectly soluble in 

 solution of potash even when very dilute, and 

 if the quantity of fibrin dissolved be large, the 

 liquid loses its alkaline flavour. 



Vegetable albumen is found to exist very 

 abundantly in the juices of most plants, and 

 still more so in nuts, almonds, and other oily 

 seeds, where it is usually associated with ca- 

 sein. It may be easily recognized by boiling 

 the expressed juice of any of the common cu- 

 linary vegetables after the fibrin has separated, 

 when it coagulates in a manner similar to ani- 

 mal albumen. It may be obtained in a tole- 

 rably pure state by boiling the filtered juice of 

 any of the leguminosce, and washing the preci- 

 pitate with alcohol and ether. It closely re- 

 sembles animal albumen in properties, and is 

 distinguished from vegetable fibrin by its so- 

 lubility in water, and from vegetable casein by 

 coagulating when heated. 



Vegetable casein has also been called legu- 

 mine, from the circumstance of its being found 

 most abundantly in the leguminosce y though it 

 is by no means confined to that tribe of plants: 

 it is also present in considerable quantity in 

 company with albumen in most of the oily 

 seeds, and in the juices of most nutritious 

 vegetables. It may be obtained by the follow- 

 ing process. Peas or beans should be soaked 

 in moderately warm water for some hours until 

 they are sufficiently soft to allow of their being 

 mashed in a mortar : the pasty mass is then 

 mixed with a large quantity of water, which 

 dissolves the casein, and thrown upon a cloth 

 to filter. The starch passes through the filter 

 together with the solution of casein, and if 

 allowed to stand, gradually subsides to the 

 bottom : when the liquid is clear, it is decanted 

 by means of a syphon, and slightly supersa- 

 turated with acetic acid, which determines the 

 precipitation of the casein in an impure state, 

 but readily purified by washing with alcohol 

 and ether. Vegetable casein resembles that 

 obtained from milk in most of its properties ; 

 gives the same insoluble skin when heated in 

 contact with the air ; and is precipitated from 

 its aqueous solution of alcohol and several of 

 the metallic oxides : it is also thrown down by 

 both vegetable and mineral acids, redissolving 



